Today, not only mobile telephones but also other mobile devices such as PDAs (Personal Digital Assistants), notebooks, etc., exchange data with wireless networks via radio interfaces. Typically, a radio base station of a network, e.g. a mobile network, serves the mobile device by routing data received from the device through the network towards the recipient, and by transmitting data received from the network side over the radio interface towards the mobile device.
The transmission resources available over the radio interface, such as frequency (bandwidth), time (timeslots available in transmission frames) and transmission power, are generally limited and therefore have to be used as efficiently as possible. In this respect, the base station controls not only the resource parameters for downlink transmissions (from the base station to the device), but also for the uplink transmissions (from the device to the base station). For the uplink, the base station has to ensure that the mobile device is synchronized with the transmission scheme of the radio interface with appropriate accuracy to avoid waste of resources. To this end the base station analyzes received uplink signals, derives appropriate adjustment values for the uplink transmission parameters used by the device and sends information indicating the necessary adjustments towards the mobile device, which then has to adjust its transmission parameters accordingly.
As an example, the radio base station determines timing misalignments between the mobile device and the radio base station. Timing misalignments are caused by the variable propagation round trip delay resulting from a changing distance between mobile device and base station as well as from the mutual drift between the clocks in the base station and the device.
Whereas the synchronization of the mobile device may be performed in a straight-forward manner in case of an established uplink connection, during which signals from the device are continuously received and analyzed at the base station, no such analysis is possible in case the device wants to connect for the first time (for example at power-up or during a handover) or from a standby status (in which the device only listens to the downlink). In these circumstances a random access procedure has to be performed to achieve synchronization.
In networks such as mobile GSM or UMTS networks, a physical random access channel (RACH) is provided by the base station (also called Node-B in UMTS) over the radio interface which allows a mobile device to perform a random access procedure. During this procedure, the mobile device transmits a specific access burst (as opposed to normal transmission bursts) in the RACH. In case of a successful detection and analysis of the access burst, the base station responds by transmitting proper adjustment parameters to the mobile device.
When transmitting the access burst, the uplink transmission parameters such as time, frequency and power are in general not accurately aligned with the transmission scheme supported by the radio base station. Therefore additional resources have to be provided to the random access channel to allow for misalignments and avoid interference of the random access bursts with well synchronized normal bursts transmitted, for example, in neighbouring time slots. These extra resources comprise, for example, guard periods and guard bands in the time and frequency dimension, respectively.
In GSM networks, a particular RACH time slot is defined in the time domain. For example, time slot or sub-frame 0 in each radio frame may be reserved for the RACH. In this way, the RACH is orthogonal to other data channels, e.g. traffic channels. Within a RACH, collisions may occur as multiple mobile devices may simultaneously request access. In GSM, at most one of the simultaneously received access bursts can be successfully detected, the other bursts therefore remain unanswered by the base station. A contention resolution scheme may thus include a random back off procedure, wherein the mobile devices repeat their access requests after a randomly determined time period in case of no response from the base station.
An access burst may contain a “preamble” or “signature” sequence, which is basically a sequence of symbols. Each of the symbols in turn may comprise a sequence of bits, e.g. 4 bits. Different preambles may be provided to the mobile devices to allow simultaneous access requests of multiple devices in the same cell. An access requesting device is expected to choose (e.g., randomly) one of the predetermined preambles. The detection of access bursts in the base station thus relies on searching for the occurrence of any one of the predetermined specific preamble sequences in the RACH. A specific preamble detector may be provided in the base station which comprises a number of digital filters, one filter for each of the allowed preamble sequences. In case a signal received in the RACH matches with one of the filters to at least a predetermined accuracy, an access burst can successfully be detected.
As an example, six different preambles may be used for the access procedure. In this case, six filters have to be provided in the preamble detector. Any signal received in a random access channel has to be analyzed by all six filters in parallel in order to determine if none, one or more access bursts have been transmitted. It is clear already from this simple example that the detector requires a highly complex circuitry including a plurality of digital filters operating in parallel in order to analyze the received signal. Generally, with an increasing number of admissible preamble sequences to detect, the number of filters to be provided and thus the computational complexity increases further.
The upcoming successor of the current UMTS standard called LTE (Long Term Evolution) will utilize OFDMA (Orthogonal Frequency Division Multiple Access) as an orthogonal transmission scheme. Also in this system, there will be mutual interference between access bursts simultaneously transmitted by different mobile devices. At the same time, presumably the number of simultaneous access attempts to be processed in parallel will increase and thus the computational complexity of the detector.
In non-orthogonal systems such as W-CDMA, the RACH is shared with other uplink channels. Here, the preamble detector has to cope with mutual interference not only between multiple access bursts, but also between access bursts and other bursts, e.g., normal bursts. Also in this scenario, a detection of access bursts with an appropriate confidence level requires a very complex detector.
There is thus a need for an efficient technique for detecting one or more access bursts in a random access channel which also allows construction of detectors with limited complexity.